Taxonomic Comments

Isolates of coccoid ammonia-oxidizers were first described by Winogradsky (1892, 1904). One of these strains has been named Nitrosococcus nitrosus by Migula (1900). However, this type species of the genus Nitrosococcus was not preserved in culture. Only data on the cell shape and size are available from the original description (Winogradsky, 1892). Since this species cannot be identified with certainty, it is a candidate for placement on the list of rejected names.

Another coccoid ammonia-oxidizer, originally placed in Ni-trosococcus as N. mobilis (Koops et al., 1976), has been proved via phylogenetic analyses to be a member of the genus Nitrosomonas (Woese et al., 1984b; Head et al., 1993; Teske et al., 1994).

The remaining two described species of the genus Nitrosococ-cus, N. oceani and "N. halophilus", are both members of the Gam-maproteobacteria. They are closely related to each other, as demonstrated by DNA-DNA hybridizations. Using the S1 nuclease technique, a DNA similarity of 9-10% was estimated (Pommer-ening-Roser, 1993).

In the future, the taxonomically revised genus Nitrosococcus should be reserved for representatives of ammonia-oxidizing bacteria belonging to the Gammaproteobacteria and being closely related to the grouping of N. oceani and "N. halophilus".

The cells are spherical to ellipsoidal, 1.8-2.2 |m in diameter. Cells generally occur singly or in pairs, but sometimes aggregates are formed via production of exopoly-meric substances. Motile cells possess 1-20 flagella. Cells have a typical Gram-negative envelope, with two additional cell wall layers composed of subunits arranged in rectilinear and hexagonal arrays. Cells possess an extensive intracy-toplasmic membrane system, arranged as a centrally located stack of parallel, flattened vesicles. Cells have an obligate salt requirement, with optimum NaCl concentrations for growth of 400-500 mM. The maximum concentration of ammonium salts is around 1000 mM at pH 7.8. Urease positive. All strains have been isolated from marine environments.

Cells are spherical to ellipsoidal, 1.8-2.5 |m in diameter. Gram-negative cell wall. Additional outer layers, as demonstrated for N. oceani, may exist, but have not been reported. Motile cells have a tuft of flagella. Intracytoplasmic membranes are arranged as a stack of centrally located flattened vesicles. Cells are obligately halophilic, optimum NaCl concentrations for growth are 600-800 mM, up to 1600 mM is tolerated. Ammonium compounds are tolerated up to concentrations around 600 mM. Urease negative. Isolates originate from salt lagoons and salt lakes. The mol% G + C of the DNA is: 50.5 (Tm). Deposited strain: Nc 4.